5.4 Pressure

Question 1

Pressure

Answer 1

The concentration of a force or the force per unit area.
P = F/A

P = pressure (pa)
F = Force (N)
A = area (m^2)

Question 2

Atmospheric pressure

Answer 2

The atmosphere is a thin layer of air around the Earth, and it becomes less dense as altitude increases. Air molecules create atmospheric pressure by colliding with surfaces. As you go higher, there are fewer air molecules above, so the pressure decreases with altitude.

Question 3

Liquid pressure

Answer 3

In a liquid, the pressure at a point increases with the height of the column of liquid about that point.
If there is more liquid above that point, then the pressure is more.
This is because the pressure in a liquid is caused by the weight of the liquid pushing against objects immersed in the liquid.
As the liquid becomes deeper, the amount of liquid (and hence the weight) increases which causes the pressure to increase.
p = ρhg

p = pressure in pascals (Pa)
h = height of the column in metres (m)
ρ = density of the liquid in kilograms per metre cubed (kg/m3)
g = gravitational field strength on Earth in newtons per kilogram (N/kg)

Question 4

Upthrust

Answer 4

A partially submerged object (an object that’s not fully in a liquid) will experience greater pressure on the bottom surface than on the top surface. This creates a resultant force upwards. We call this force upthrust.
The upthrust that acts on an object is equal to the weight of the liquid that has been forced away (displaced) by that object.
If the object’s weight is equal to the upthrust, then the forces balance and the object will float in the liquid.
If the object’s weight is greater than the upthrust, then the object will sink.